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I am preparing for my aerodynamics exam and I'm having trouble with this question.

Why is the Reynolds number of the airflow over a wing important in determining the value of lift and drag

The Reynolds number is important for determining laminar and turbulent flows over a body. It is a dimensionless number and expresses

the ratio of inertial forces to viscous forces. A Reynolds number less than 5 x 10^{5}is likely to be laminar whereas a Reynolds number greater

than 5 x 10^{5}will most likely be turbulent.

The Reynolds number is important for determining drag as we can use it to calculate the length of the laminar and turbulent flow

over a wing. Moreover we can use this to calculate the friction coefficient (skin friction drag) acting on the wing.

Laminar Flow

Cf = [itex]\frac{1.328}{\sqrt[]{}Re}[/itex]

Turbulent Flow

Cf = [itex]\frac{0.074}{\sqrt[]{}Re^{2}}[/itex]

It is important for calculating lift because as Reynolds number increases, the maximum lift coefficient increases. But this does not occur

indefinitely; when flows become very turbulent, the maximum lift coefficient begins to drop and so does the overall lift coefficient.

Re = [itex]\frac{ρV×}{\mu}[/itex]

ρ = Density

V = Velocity of the free-stream airflow

× = The characterised length of the aerofoil

[itex]\mu[/itex] = The fluid viscosity

I would like to know if the way I have explained the Reynolds number is correct particularly with reference to lift, and is the Reynolds number important when

dealing with other types of drag?

Thanks!

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# Homework Help: Reynolds number and the relationship with lift and drag.

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